With the recent progress in the digital technology, analog signals such as audio or video signals are frequently subjected to sampling, quantization and encoding for transmission, recording or reproduction in the form of what is called pulse code modulated signals (PCM signals).
In converting and transmitting analog signals into and as PCM digital signals, it is well known that the bandwidth of the transmissible analog signals becomes broader with increase in the sampling frequency whereas the dynamic range becomes broader with increase in the number of quantizing bits. Hence, when it is desired to perform digital transmission of the original analog signals with high fidelity, that is, with a wide band and a large dynamic range, a higher frequency and a larger number of quantization bits are required, with the result that the number of bits transmitted per unit time or what is called the bit rate is increased.
However, the properties of the transmission medium inclusive of the recording medium and those of the digital signal processing speed at the transmission and reception sides (recording and reproducing sides) result in limitations on the bit rate. As a matter of fact, from the economic considerations and in view of the cost performance of the PCM signal recording and/or reproducing apparatus, it is imperative to make high quality signal transmission, recording or reproduction with as low a bit rate as possible.
Such differential PCM system or additive PCM system is known in the art as the technology for transmitting signals with large dynamic range at a relatively low bit rate. These systems are subject to error propagation. When it is desired to provide for some error corrective capacity, there results an increase in redundancy so that the effect of bit rate reduction is lowered.
The present inventors have devised, in the Japanese Patent Application No. 97687/1983, a blockwise digital signal transmission method wherein plural words of the differential PCM mode data compose one block, and at least one sampled value data word is arranged in each block. Owing to the presence of the sampled data word even when an error is introduced into the differential or additive PCM data, error propagation is confined within the extent of the block so that error propagation is allowed to continue for only a limited time interval.
The present inventors have also devised in the Japanese Patent Application No. 97688/1983 a digital signal transmission system wherein the one mode of a maximum data compression ratio per each block from the normal PCM mode (straight PCM mode), the differential PCM mode and the additive PCM mode is selected and the data of each block is sequentially transmitted with the selected mode, so that the occasionally maximum transmission efficiency may be obtained per each block for achieving signal transmission, recording or reproduction with a low bit rate and a high quality.
The present inventors have also devised a digital signal transmission system, in the Japanese Patent Application No. 97689/1983 wherein the noise spectrum which depends on frequency spectrum of the input signal is obtained by the noise shaping in the best use of what is called the masking effect, so as to reduce the apparent noise.
According to the Japanese Patent Application No. 97689/1983, the maximum absolute values of, for example, the differential PCM data and the normal or straight PCM data, are compared to each other, the mode with the occasionally lesser value is selected, and the error feedback value at the time of the noise shaping process is switched as a function of mode selection so as to control the noise spectrum. Assuming that the input signal is a sinusoidal wave, the frequency f.sub.T at which the switching takes place between the differential and straight PCM modes is one-sixth of the sampling frequency f.sub.s (f.sub.T =f.sub.s /6). For example, f.sub.T =5.3 kHz for f.sub.s =32 kHz. That is, the differential PCM mode is selected for the input signal frequency up to ca. 5.3 kHz, and the straight PCM noise is selected for the higher frequency.
In the apparatus shown in the Japanese Patent Application No. 97689/1983, it has been found that the low frequency noise becomes more audible when the differential PCM mode is selected for an input signal of the mid range frequency slightly lower than the above defined frequency f.sub.T.
This is because the low frequency noise becomes difficult to mask when the frequency of the input signal is in the mid range, and also because the noise energy of the differential PCM mode is increased with rise in the frequency of the input signal. It has also been found that the noise due to coding error is also increased for the input signal as the above.
It is therefore an object of the present invention to provide an apparatus for transmitting digital signal wherein the low frequency noise can be suppressed for the mid range frequency of the input signal so that the masking effect, and works more efficiently the apparent noise and the noise due to coding error may be reduced.